Mig's cabs eh?!
Interesting conversation this. My own views are that you might get away with two pints depending on your body type, the strength of the tipple, the amount of time it was drunken over, if food was eaten at the same time. And that's the rub, by setting an ambiguous limit like the current one then people will play the numbers game, so set a next to zero amount and make it clear that drinking and driving on busy fast roads is just not acceptable today full stop.
A previous next door neighbour regularly used to drink 5 or more pints a night and drive, and I found this out after accepting a lift with them as they drove at high speed as they were late, I could not wait to get out of the car I can tell you, and never again. In saying that, my own father used to drink and drive regularly, all night sessions at the Potters Bar Conservative club (Labour man though!) and him and my mother then driving home as the sun came up were not unheard of, neither was helping my uncle into his car and pointing him in the general direction of home. These stories filled me with horror, but the only time my dad ever had an accident is when a Metro bus took off the side of his Ford Zodiac as he went around the roundabout near Shipley after dropping me off at school, and he was sober at the time. I guess it that it was socially acceptable back then (70s), added to the fact that roads were quieter, traffic slower but it wasn't right, but I don't know otherwise how he got away with it, and I don't mean not being caught by the rozzers.
On a side note, the A9, that road is dangerous as hell, and not because of the speeding, the caravans, the lorries, or Mable on the way to the post office in a 1981 Chevette that just does 40mph no matter, but the lack of decent overtaking opportunities and the inevitable frustration and chances taken caused. Though, I must say that the scenery is nice, unless it's dark and your mates Tigra with a red top Astra engine shoehorned has just burst into flames due to bad wiring by an auto electrician and you end up having to tow it for 90 miles to Loch Laggan...